Flight management systems provide assistance in following or ensure the automatic following of a route defined in the flight plan by a succession of geographic points called “waypoints”, which are associated with flight constraints and mark turns (turning points), vertical transitions or changes of guidance instructions. For this, they generate, at the start of a mission, a flyable path that follows the route logged in the flight plan and modifies it as necessary during the mission to take account of environmental conditions (weather, wind, etc.) that might have changed.
To determine the moments when the vertical and lateral transitions must be executed, and the moments when guidance instructions change or when monitoring or action messages should be displayed, the flight management systems need to know at all times the position of the aircraft relative to the points marking the vertical or lateral transitions, or changes of guidance instruction along the flyable path that has been generated. For this, they use comparisons of curvilinear distances, measured from a particular point: the destination of the aircraft or a next waypoint to be reached, following the flyable path generated previously.
The curvilinear distances from which the flight management systems get their bearings are subject to value jumps with various causes: adjustments to the path, made en route, to take account of changes of environmental conditions, unavailability and instability of the measurements (geographic location, altitude, speed, etc.) originating from flight instruments of the aircraft from which they are generated, and so on.
In practice, the new inclusion of an environmental condition unpredicted when creating the flight plan, such as a change in the direction or the force of the wind, can be reflected in a change of path (more or less tight turn at a turning point) affecting the distances to be travelled.
Similarly, in periods when the measurements needed to generate the curvilinear distances are unavailable, the flight management systems can temporarily replace them with estimates based on the elapsed time and the latest valid measurements. When measurements become available again, value jumps can occur when replacing the estimate with the value actually computed from current measurements that are once again valid.
Finally, the instabilities affecting the measurements supplied by the flight instruments, in particular on unexpected changes of sensor or measurement method (replacing a position measurement from a satellite positioning receiver with that from an uncorrected inertial unit or vice-versa, replacing an altimetric measurement from a baro-altimeter with that from a radio-altimeter, or vice-versa, and so on) are reflected in value jumps on the curvilinear distances that are deduced from them.
These value jumps affecting the curvilinear distances are seen by the flight management systems as the consequences of movements of the aircraft made at speeds that depart from the scope of the performance levels of the aircraft for which they were designed. In the presence of these, the flight management systems temporarily behave erratically which can be reflected in the detection of false violations of the flight constraints, for example minimum altitude constraint approaching a landing field, leading to an erroneous perception of the context leading the crew to make pointless or even dangerous manoeuvres.